Most oil and gas wells are drilled with a drill string comprised of drill pipe. After reaching a certain depth, the drill string is removed and casing is lowered into the wellbore. A cement valve, is normally attached to the lower end of the casing. The cement valve allows cement to be pumped down through the casing and up the annulus surrounding the casing, and prevents backflow of cement from the annulus back into the casing. Another type of casing string, referred to as a liner, may be installed in a similar manner. A casing string extends all the way back to the upper end of the well, while a liner string is hung off at the lower end of a preceding string of casing.
In another drilling technique, the casing is used as part or all of the drill string. The bit may be attached to the lower end of the casing string permanently, in which case it is cemented in place. Alternatively, it may be retrieved after reaching desired depth, such as by using a wireline, drill pipe, or pumping the bit assembly back up the casing. While drilling, the casing string may be rotated by a gripping mechanism and a top drive of the drilling rig. With liner drilling, the liner string serves as the lower end of the drill string, and a string of drill pipe is attached to upper end of the liner string.
In casing and liner drilling, if the bottom hole assembly, which includes a drill bit and optionally measuring instruments and steering devices, is to be retrieved before cementing, the operator will install a cement valve at the lower end of the liner after retrieval of the bottom hole assembly. The cement valve may be lowered into place on a wire line or a string of drill pipe and locked to a profile at the lower end depth of the liner string. Also, it is has been proposed to pump the cement valve down the casing, rather than convey it on a wire line. The cement valve may have a flapper valve to prevent back flow of cement. It may also have a frangible barrier to allow the cement valve to be pumped down the casing string. Once in place, increased fluid pressure causes the barrier to break and the fluid to flow out the lower end of the cement valve.
It has also been proposed to pump a receptacle down the casing string and latch it into a profile at the lower end prior to cementing. The receptacle has a passage that allows the downward flow of cement, but does not have a valve to prevent backflow. At the conclusion of cementing, a wiper plug or prong is pumped down into engagement with the receptacle. The prong stabs into the upper end of the receptacle to form a seal and retain the plug to prevent backflow of cement.
After the cement is cured, if the operator intends to drill the well deeper, the drill string must drill through the receptacle and wiper plug. It is thus desirable to make the receptacle and wiper plug of easily drillable materials. These materials must meet the requested specifications of the tools.